In injecting a thermoplastic resin into a cavity of a mold, improvement of reproducibility in imparting the mold surface profile on the molded article and improvement of gloss of the molded article can usually be achieved to some extent by making a proper selection of molding conditions, for example, by increasing the resin temperature, the mold temperature or the injection pressure.
Of these factors, a mold temperature is the most influential. The higher the mold temperature, the better the appearance of the resulting molded article. However, an increase in mold temperature makes a time required for cooling the plasticized resin longer, leading to a reduction in molding efficiency. Therefore, it has been demanded to develop a molding technique for achieving improved mold surface reproducibility without increasing the mold temperature or, even if the mold temperature is increased, without requiring an extension of a cooling time. A method comprising introducing a heat transfer medium and a coolant alternatively into the respective holes of a mold to make a heating and cooling cycle has been adopted. However, this method involves high consumption of heat and extended heating and cooling times, making the molding cycle time longer.
Many reports have been made to date on a method for improving mold surface reproducibility comprising coating a cavity wall of a mold with a substance having a small thermal conductivity, i.e., a heat insulating layer. For example, WO 93/06980 suggests use of polyimide as an insulating layer, and JP-A-54-142266 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses the use of an epoxy resin as an insulating layer.
The merit of injection molding and the like resides in that an article of complicated shape can be obtained through a single shot. Therefore, the molds used in these molding methods usually have a complicatedly shaped cavity. On the other hand, the heat insulating layer of a heat insulating layer-coated mold has a thickness of about 0.1 to 0.2 mm for general injection molding but, under some molding conditions, the heat-insulating layer for use even in injection molding should have a thickness of about 0.2 to 0.5 mm. For blow molding, the insulating layer should have a thickness of not less than 0.3 mm and, in some cases, 0.4 mm or more. It has been demanded to uniformly and economically provide such a thick heat insulating layer on a cavity wall of complicated shape.
Uniform formation of a thick heat insulating layer on a cavity wall having a complicated shape has conventionally been carried out by repeating formation of a thin coating layer by spray coating or brush coating followed by hardening by, for example, heating several times or, in some cases, several tens of times until a desired thickness is reached. If a cavity wall of complicated shape is coated thick at a time, the coating sags during application, making it difficult to provide a coating layer of uniform thickness on the cavity wall of complicated shape. A quantitative sag (Q) is generally considered to be proportional to the thickness (t) of a coating film immediately after being applied, being represented by the following equation: EQU Q=d.multidot.g.multidot.t.sup.3 /.eta.
wherein Q represents a sag amount; d represents a specific gravity of a coating; g represents an acceleration of gravity; and .eta. represents a viscosity of the coating; and t represents a thickness of a coating film.
In order to apply a coating to a uniform thickness while minimizing a sag amount (Q), it is necessary to reduce the amount of the coating applied at a time to reduce the coating thickness (t). In other words, the steps of applying a coating thinly and solidifying by heating, crosslinking, etc. had to be repeated many times to increase the coating thickness stepwise. In order to form a coating film as thickly as possible at a time without causing sags, it is desirable that the coating to be applied by spray coating or brush coating has a reduced solvent content, i.e., a high concentration. However, a highly concentrated coating generally has a high viscosity and is difficult to apply uniformly.
Hence, a coating which has a high concentration and yet has a low viscosity is preferred. From this viewpoint, a coating solution comprising a low-molecular weight substance which can be converted to a high-molecular weight substance upon reaction after application is preferred. Thermosetting resins, such as epoxy resins, are extremely favorable coatings in this aspect. However, a cured epoxy resin is generally brittle with a small elongation because of involvement of crosslinking. Therefore, if applied to a mold cavity wall made of metal, it tends to undergo cracking under the heating/cooling cycles of molding. Further, a heat insulating layer covering the mold surface is also required to have heat resistance. It is necessary to improve heat resistance and toughness of a cured epoxy resin which are conflicting with each other, in case of using a cured epoxy resin as a heat insulating layer.
A method of adding rubber of various kinds, such as nitrile rubber, to an epoxy resin has been in long use for making a cured epoxy resin tough. However, the effect of addition of rubber is hardly manifested in a heat-resistant cured epoxy resin having a high crosslinking degree, i.e., a small molecular weight among crosslinking bonds. Further, it is preferable that the heat insulating layer formed on a mold surface be easy to be polished to provide a mirror surface.
What is required of a heat insulating layer is to have satisfactory coating properties in providing a large coating thickness, to have heat resistance and elongation at break to secure durability against molding, and to have hardness and polishability to give a mirror surface. Thus, it has been demanded to provide a mold coated with a heat insulating layer having improved heat resistance and improved toughness while retaining the excellent workability possessed by a thermosetting resin, such as an epoxy resin.